The present invention pertains to a printing apparatus and printing method for printing images through formation of monochrome or multi-color dots on a recording medium during main scanning.
An inkjet printer is used as a device for outputing images processed by a computer or images captured by a digital camera. An inkjet printer forms dots by ejection of ink of various colors such as cyan, magenta, yellow and black, for example. Dots of each color are typically ejected from a print head while the print head is moving in a main scanning direction. If the positions at which the dots of each color are formed are misaligned, it would cause a problem of reduced image quality.
This problem of image quality deterioration due to dot formation misalignment occurs in both uni-directional recording and bi-directional recording. Here, uni-directional recording refers to a recording method in which, where the print head moves back and forth along the main scanning passes, the dots are ejected only when the print head is moving along one of the passes. Bi-directional printing refers to a recording method in which dots are ejected when the print head is moving along both of the main scanning passes. While the problem of dot position misalignment typically occurs with respect to dots of different colors in uni-directional printing, it occurs in bi-directional printing with respect to dots of the same color formed during forward and reverse passes.
In the conventional printer, the dot position misalignment may be reduced by adjusting the formation positions of color dots in the main scanning direction while using black dots as a reference, for example. This type of dot position misalignment adjustment is realized by a head drive circuit that supplies drive signals to the print head while changing the output timing of the drive signals.
However, the above-described conventional dot position misalignment adjustment method has various inherent limitations. For example, because the drive signal timing can be changed only for the entire print head in a typical printer, dot position misalignment adjustment is limited to what can be achieved by the timing change.
The present invention was made in order to resolve the abovementioned problem with the conventional art, and an object thereof is to provide the technique that reduces the dot position misalignment in the main scanning direction using a means other than changing the drive signal output timing from the head drive circuit, thereby improving image quality.
In order to attain the above object, in the present invention, while performing main scanning in which a head having a plurality of nozzles that eject ink is moved in prescribed forward and reverse directions relative to a print medium, sub-scanning is carrying out in which the print medium is forwarded in a sub-scanning direction perpendicular to the main scanning direction relative to the head. The head is driven in accordance with print data along at least one of the forward or reverse scanning passes. Dots are formed in at least some of the a plurality of pixels aligned in the main scanning direction. The dot formation position misalignment for each nozzle in the main scanning direction are corrected using image pixel value data indicating a dot formation status regarding image pixels that constitute images, as well as adjustment pixel value data that indicates existence of adjustment pixels in which dots are not formed and are used to adjust positions of the image pixels in the main scanning direction. In this arrangement, as dots are formed in accordance with the print data, the misalignment of the formation positions of the dots from each nozzle in the main scanning direction is corrected using (i) image pixel value data indicating the dot formation status in image pixels that comprise the image, and (ii) adjustment pixel value data indicating the existence of adjustment pixels in which dots are not formed and which are used to adjust positions of the image pixels in the main scanning direction. Various aspects of the present invention will be explained below.
(1) Allocation of Adjustment Pixels at Either End of Main Scanning Direction
First, the allocation of the adjustment pixels are set to one or both ends of the image pixel value data so that the amount of the dot formation position misalignment is corrected. Here, the xe2x80x98allocation of adjustment pixels to one or both endsxe2x80x99 may include the case in which adjustment pixels are not allocated at one end. Raster data is generated from the image pixel value data and the allocation of the adjustment pixels. The raster data has the image pixel value data and the adjustment pixel value data placed at least one side of the image pixel value data. The print data including the raster data is then generated. The head is thereafter driven in accordance with the print data while main scanning is being performed.
According to this aspect of the present invention, the misalignment of the dot formation positions can be corrected and high-quality printing can be realized by giving the following characteristics to the print data for driving the head. Typically, print data includes those multi-level data for each of pixels arrayed in a predetermined number, which are converted from image tone values. This multi-level data corresponds to the image pixel data in the present invention. The print data in the present invention contains, in addition to the image pixel data, data regarding a prescribed number of adjustment pixels in the main scanning direction. The adjustment pixel data represent the blank left and right margins in the main scanning direction.
Through the use of print data having this structure, the printing apparatus of the present invention can correct dot formation position misalignment within the range attained by the adjustment pixels. An example will be described in which main scanning is performed from left to right. Assume that the head includes a nozzle that forms dots to the left of the target pixel position due to its ink expulsion characteristic. In the printing apparatus of the present invention, the amount of dot formation misalignment attributable to the nozzle is stored beforehand. Here, the amount of misalignment is assumed to be one pixel. In the present invention, the position at which a dot is formed by this nozzle is shifted in accordance with this stored misalignment amount, and print data is generated accordingly. In other words, print data is generated in which a dot is formed at a position that is shifted to the right by one pixel from the target pixel position. This is equivalent to setting the adjustment pixel allocation such that the number of adjustment pixels on the right side is reduced by one and the number of adjustment pixels on the left side is increased by one in the main scanning direction, relative to those in the case in which the dot could be formed at the correct position. When ink is ejected from this nozzle based on this print data, the abovementioned dot formation shift occurs, and a dot is formed at the pixel on which it should be.
In the printing apparatus of the present invention, dot formation position misalignment may be corrected in pixel-width increments based on this principle. In recent years, pixel width in the main scanning direction has become extremely small, and it has become possible to sufficiently correct for dot formation position misalignment for each nozzle by shifting the dot formation position in pixel-width increments. Therefore, high-quality printing may be attained with the printing apparatus of the present invention. Moreover, because the present invention does not require new hardware for the head driving mechanism in order to carry out the above correction, it is possible to reduce the degree of dot formation position misalignment with relative ease.
In the present invention, the print data may be generated in various steps. For example, print data may be generated in two steps comprising a first step wherein basic data is generated in which a prescribed number of adjustment pixels are located at opposite ends of the image pixels along the main scanning direction, regardless of the amount of dot formation position misalignment, and a second step wherein the image pixels position is shifted in accordance with the amount of dot formation position misalignment, i.e., the allocation of adjustment pixels at both ends is changed.
Alternatively, print data may be generated in two steps comprising a first step wherein the allocation of adjustment pixels at opposite ends of the image pixels is specified in accordance with the amount of dot formation position misalignment, and a second step wherein adjustment pixels are added to the opposite ends of the image pixels pursuant to the specified allocation.
Furthermore, in the printing apparatus of the present invention, the number of adjustment pixels may be set to any appropriate value within the range that enables dot formation position misalignment to be corrected. This value may be one or more.
In the present invention, the allocation of adjustment pixels in accordance with the formation position misalignment amount may be carried out individually for each nozzle, but where ink of a prescribed color is ejected from each nozzle to form dots of various colors, the allocation is preferably set separately for each ink color.
In this aspect of the present invention, the dot formation position misalignment is corrected separately for each color. Typically, the print head characteristics regarding dot formation position are substantially identical for each color, due to the manufacturing process and the ink viscosity. Therefore, dot formation position misalignment may be corrected relatively easily using the arrangement described above. Furthermore, dot formation position misalignment has a significant effect on image quality when it occurs between dots of different colors. Because the arrangement described above allows such misalignment between dots of different colors to be easily reduced, it has the effect of substantially improving image quality.
Moreover, where the nozzles are classified into a plurality of nozzle rows that extend in the sub-scanning direction, and dots are formed using the nozzles in these a plurality of nozzle rows, which are themselves aligned in the main scanning direction, it is preferred that the allocation be set separately for each nozzle row. The dot formation position characteristics of the print head nozzles may be identical for all nozzles belonging to a given nozzle row. In such a case, image quality may be improved relatively easily by correcting dot formation position misalignment for each row.
The amount of dot formation position misalignment may be stored separately for each nozzle in a misalignment amount memory unit, and the allocation setting unit may have a function to set the adjustment pixel allocation separately for each nozzle. This function enables dot formation position misalignment to be corrected in a more precise fashion.
Where the image pixel value data is two-dimensional image data indicating pixels aligned in the two dimensions of the main scanning direction and the sub-scanning direction, it is preferable that adjustment pixel allocation be performed in the manner described below. The relationship between each nozzle mounted in the head and the two-dimensional image data is first determined in accordance with the amount of the sub-scanning forwarding, and the adjustment pixels are then allocated based on this determination.
Through this operation, it may be determined which nozzle will form each raster line, i.e., the pixels aligned in the main scanning direction, in the print data. The dot formation position misalignment may then be corrected based on the results of this determination. As a result, dot formation position misalignment may appropriately performed for each individual nozzle, and the quality of the printed images may be significantly improved. In a printing apparatus employing sub-scanning, because the print data is typically supplied to the head upon determination of the relationship between the raster lines and the nozzles, the determining means required to supply the print data may be employed as the determining means in the printing apparatus described above.
The generation of print data in a printing apparatus employing sub-scanning may be performed in various processes as well. For example, print data may be generated in two steps comprising a first step wherein a prescribed number of adjustment pixels are allocated at opposite ends of the image pixels regardless of the relationship of each raster line to the nozzles, and a second step wherein the raster/nozzle relationship is determined and the allocation of adjustment pixels is corrected. Naturally, it is acceptable if only image pixel data is prepared in the first step and the adjustment pixels are added in the second step.
Alternatively, print data may be generated in two steps comprising a first step wherein the raster/nozzle relationship is determined and the allocation of adjustment pixels is set, and a second step in which the adjustment pixels are added to the image pixels in accordance with the set allocation and print data is thereupon generated.
It is preferable for the head to be driven along both the forward and reverse passes of main scanning. Generally, where dots are formed along both the forward and reverse passes of main scanning, i.e., where bi-directional recording is performed, the degree of dot formation position misalignment increases. Let us consider an example in which dots are formed while the head is moving from the left to the right during forward movement, as well as while the head is moving from the right to the left during reverse movement. It will be assumed that during forward movement the dot formation position for a particular nozzle is misaligned to the left by one pixel relative to the target pixel position. Conversely, during reverse movement, the dot formation position for this nozzle is misaligned by one pixel to the right. As a result, the dot formed during forward movement and the dot formed during reverse movement are offset relative to each other by two pixels. In bi-directional recording, the dot formation position misalignment has a major effect on image quality as described above. Therefore, by applying the present invention in a printing apparatus that performs bidirectional recording, the dot formation position misalignment can be relieved, and the resulting improvement in image quality is striking.
The head may also be driven only along either the forward or the reverse scanning pass. Using this method enables the problem of dot formation position misalignment caused by the scanning in different directions to be avoided.
Where dot recording is concerned, it is preferred that the dot recording for each main scanning line be completed during one pass of the head. When this feature is adopted, each raster line is created by a single nozzle, and therefore dot formation position misalignment may be corrected relatively easily and with high precision. Incidentally, there is a so-called overlap method where each raster line is formed with a plurality of nozzles during recording. In the overlap method, odd-numbered pixels on a raster line are recorded by a first nozzle, and even-numbered pixels are recorded by a second nozzle after the recording medium is fed forward during sub-scanning. When this type of recording is performed, a single raster line is formed using two nozzles having different dot formation position characteristics. Therefore, the operation by which to correct dot formation position misalignment is exceedingly complex. On the other hand, where each raster line is formed using a single nozzle, the adjustment pixel allocation may be easily set for each raster line, allowing dot formation position misalignment to be carried out with relative ease. However, this does not mean that the present invention cannot be applied to the overlap method.
The present invention does not require that misalignment correction be carried out over the entire image data. Misalignment correction may be carried out only in areas in which dot misalignment has a significant effect on image quality. For example, misalignment correction may be omitted for dots of an ink color with relatively low visibility. It is also acceptable if misalignment correction is carried out only in areas in which dot misalignment has a significant effect on image quality, such as areas in which dots are formed with an intermediate level of recording density. If misalignment correction is carried out only where dot misalignment has a significant effect on image quality as described above, the burden on the processor during printing can be reduced and the speed of processing can be increased.
Prescribed test patterns designed to enable detection of the amount of dot formation position misalignment for each nozzle may be printed, and the amount of dot formation position misalignment may be subsequently specified based on these test patterns.
The amount of dot formation position misalignment depends on various factors, such the ink expulsion characteristic of each nozzle, the amount of backlash during the forward and reverse movement of the head, and changes in various factors such as the viscosity of the ink. Consequently, dot formation position misalignment can occur even after the product is shipped. Accordingly, the amount of misalignment may be specified by printing out test patterns and setting the amount of misalignment based on these test patterns. Therefore, even where dot formation position misalignment occurs after shipment, the user can relatively easily reset the misalignment amount stored in memory. As a result, high-quality printing may be relatively easily maintained, and the ease of use of the printing apparatus may be improved.
Various methods may adopted for the setting of the misalignment amount based on the test patterns. For example, the misalignment amount may be specified using a method of printing test patterns in which dots are formed at various pre-set timings and selecting the timing offering the best dot formation positions.
(2) Reversal of Placement of Adjustment Pixels on Occurrence of Prescribed Event
The present invention may also be used in the following fashion. First, print data including raster data, sub-scan feed data and adjustment pixel placement data are generated. Here, raster data block has at least the image pixel value data with regard to each nozzle for each main scanning session. Sub-scan feed data indicates a feed amount-for the sub-scanning performed after each main scanning session. Adjustment pixel placement data, that is separate from the raster data block, indicates numbers of adjustment pixels to be placed at opposite ends of the image pixel value data. The adjustment pixel placement data functions as at least a part of the adjustment pixel value data. The head is thereupon driven and dots are formed in both the forward and reverse scanning passes in accordance with the print data. When a direction of a scheduled pass for each raster data block is reversed, the reversal is detected. The raster data block is reconstructed by reversing placement of the adjustment pixels across the image pixels sandwiched between the adjustment pixels, for the raster data block regarding which the pass is reversed, and by aligning, based on the reversed placement of the adjustment pixels, the adjustment pixel value data at least one of the opposite ends of the image pixel value data.
Through this operation, dot formation misalignment may be appropriately corrected with regard to raster data to be recorded in a scanning direction reversal from the scanning direction assigned initially.
The raster data may include, as at least a part of the adjustment pixel value data, adjustment pixel data having the same format as the image pixel value data. In this arrangement, the printing unit that receives the print data can process the image pixel value data and the adjustment pixel data as a single block of pixel data, making processing simpler.
It is preferable for raster data to include a directional flag indicating the direction of the scheduled scanning pass for each raster data block. In this arrangement, the printing unit can know which scanning direction is allocated to the printing of each rasterline of the raster data.
Where a process is included in which dots of various colors are formed through ejection of ink of a prescribed color from each nozzle, it is preferred that the adjustment pixel placement number in the adjustment pixel placement data be set separately for each ink color. In this arrangement, dot formation positions may be corrected in accordance with the characteristics of each ink.
Where a plurality of nozzles are classified into a plurality of nozzle rows that extend in the sub-scanning direction and that are aligned in the main scanning direction, and dots are formed using the nozzles in these a plurality of nozzle rows, it is preferred that the adjustment pixel placement number in the adjustment pixel placement data be set separately for each nozzle row. Because the nozzles in a nozzle row have common characteristics, dot formation position misalignment may be corrected properly by this independent setting.
It is furthermore preferred that the adjustment pixel placement number in the adjustment pixel placement data be set separately for each nozzle. Because dot formation position misalignment may be corrected for each nozzle, the quality of the resulting printing will be improved.
(3) Dot Formation Using a Plurality of Base Drive Signals
Printing is sometimes performed in the following manner. First, a plurality of base drive signals are generated in which signals for the nozzles to record one pixel are repeated. Here, the a plurality of base drive signals have same periods but different phases that are mutually offset from each other. Drive signals to drive the driving devices mounted in each nozzle to eject ink are generated from the base drive signals to form dots.
In this case, it is preferred that the image pixels and the adjustment pixels aligned in each main scanning line are classified into a plurality of pixel groups when the print data is generated. Dots on respective pixels in the a plurality of pixel groups are formed based on the different base drive signals respectively.
When this process is followed, dots can be recorded in accordance with a higher pixel density than is possible when dots are formed using a single base drive signal. Moreover, even where the placement of the adjustment pixels varies based on the dot formation position misalignment, this can be taken into account when dot recording is carried out.
Where the a plurality of base drive signals includes N base drive signals having phases that are sequentially offset by an amount equal to 1/N of one period (N being a natural number equal to or greater than 2), it is preferred that the number of the pixel groups is N. In this arrangement, dot recording may be performed at a pixel density that is N times higher than would be possible where dots were formed using a single base drive signal. Moreover, because the phases of the base drive signals differ by a uniform amount, recording of an image may be carried out with a uniform pixel density.
Where the pixels are classified into a plurality of pixel groups, it is preferred that every Nth pixel of the image pixels and the adjustment pixels aligned in a main scanning line are classified into the same pixel group in the order of their placement. In this arrangement, high-quality printing may be performed using a simple and systematic process.
It is preferred that the head be driven along both the forward and reverse passes of main scanning. In this arrangement, the time required for printing may be reduced. The head may also be driven either the forward or reverse scanning passes. In this arrangement, the problem of dot formation position misalignment attributable to the different main scanning directions can be avoided.
(4) Misalignment Adjustment Performed Together with Compensation for Interval Between Nozzle Rows
Where the a plurality of nozzles classified into a plurality of nozzle rows that extend in the sub-scanning direction and that are aligned in the main scanning direction with a prescribed interval therebetween, the delay data may be used. The delay data indicates an amount of delay needed to correct for a difference in times that nozzles arrive at a particular pixel during main scanning in accordance with a design distance in the main scanning direction between the a plurality of nozzles. In this case, it is preferred that the following steps occur. First, the delay data are readjusted, so that the dot formation position misalignment amount may be corrected. Then using the readjusted delay data as the adjustment pixel value data, serial data is generated. The serial data includes the readjusted delay data and the image pixel value data that follows the readjusted delay data, for each nozzle during each main scanning session. Dots are then formed based on the serial data. In this arrangement, the delay data to compensate for the interval between the nozzles in the main scanning direction is effectively used to correct dot formation position misalignment.
For generating dots, a plurality of base drive signals may be generated in which signals for the nozzles to record one pixel are repeated. Then from the base drive signals, drive signals may be generated to drive the driving devices mounted in each nozzle to eject ink. In this case, it is preferred that the following steps occur. First, the delay data is prepared in units of one period of the base drive signals. The delay data is then readjusted in units of one period of the base drive signals based on the misalignment amount. Drive signals are then generated from the base drive signals and the serial data for each nozzle. In this arrangement, the delay data may be adjusted in units of the number of drive signals to correct dot formation position misalignment.
It is preferred that the nozzle rows aligned in the main scanning direction are aligned with an interval therebetween equal to a multiple m (m being a natural number equal to or greater than 1) of a pixel pitch corresponding to the print resolution. Dot position misalignment caused by the intervals between these nozzles may be effectively eliminated using delay data prepared in units of one period of the base drive signals.
When generating the base drive signals, N base drive signals may be generated such that they have same periods but different phases that are sequentially offset by an amount equal to 1/N of one period, and the base drive signals may be supplied to the driving devices of the nozzle group corresponding to each the base drive signal. In this case, it is preferred that the following steps occur. First, the a plurality of nozzles are classified into N nozzle groups (N being a natural number equal to or greater than 2). The drive signals are then generated from the serial data for each nozzle and the base drive signals supplied to the driving device for each nozzle. In this arrangement, dot recording may be performed at a high pixel density that is a factor of N higher than it would be when dots were formed using a single base drive signal. Furthermore, processing to correct dot formation positioning misalignment may be carried out after the image pixels are assigned to each base drive signal. Therefore, dot formation position misalignment may be carried out using less data than would be required if pixel data after the correction were assigned to each base drive signal.
In addition, it is preferred in the above configuration that the nozzle rows aligned in the main scanning direction are aligned with an interval therebetween equal to a multiple (Nxc3x97m) (m being a natural number equal to or greater than 1) of a pixel pitch corresponding to the print resolution. Dot position misalignment caused by the intervals between these nozzles may be effectively eliminated using delay data prepared in units of one period of the base drive signals, even where printing is performed with high dot density using a plurality of base drive signals.
It is preferred that the head be driven along both the forward and reverse passes of main scanning. In this arrangement, the time required for printing may be reduced. The head may also be driven either the forward or reverse scanning passes. In this arrangement, the problem of dot formation position misalignment attributable to the different main scanning directions can be avoided.
The present invention may be realized in the various aspects as follows.
(1) Printing apparatus. Printing control apparatus.
(2) Printing method. Printing control method.
(3) Computer program to implement the above apparatuses and methods.
(4) Recording medium on which is recorded the computer program to implement the above apparatuses and methods.
(5) Data signals embodied in a carrier wave, including the computer program to implement the above apparatuses and methods.